get a different helmet.
seriously though, i;m not sure that a rubies 1 or 2 piece jango can be sufficiently modified to pass as a canon jango costume. at least not without alot of $ and work. if that's the direction you're wanting to go, a new bucket would be the way to go.
however, if you're set on using the rubies jango for something, you can get a decent shape from it by lightly warming it and reshaping it. then stick it in the freezer or cold water for a couple minutes to set it. installing a craft ring in it's base and in the dome will help to hold the shape. although being a vinyl helmet, it can still suffer warping even with those installed.
